High-purity silicon is generally prepared in a multistage process proceeding from metallurgical silicon. In many known processes, for this purpose, metallurgical silicon is first converted to a silicon-hydrogen compound which can then be decomposed thermally to silicon and hydrogen. A corresponding reaction sequence is described, for example, in DE 33 11 650.
Silicon compounds are typically decomposed thermally either over particles heated to a high temperature in a fluidized bed or filaments heated to a high temperature in a decomposition or pyrolysis reactor. The latter procedure is described, for example, in EP 0181803. Detailed descriptions of suitable fluidized bed reactors can be found, for example, in EP 1 397 620, EP 1 337 463 or EP 1 343 722.
Both thermal decomposition of silicon compounds in a fluidized bed and decomposition over filaments heated to a high temperature have a crucial disadvantage, namely that it is necessary to “harvest” the precipitated silicon at regulator intervals. The reactors in which the decomposition is effected have to be shut down for this purpose so that it is possible to remove the filaments or the particles on which the silicon has been deposited and to replace them. This is associated with a high level of inconvenience and cost. First, the replacement filaments required or the replacement particles first themselves have to be produced in a complex process. Second, considerable time and energy losses are associated with the regular stoppage and restart of the pyrolysis reactors. Furthermore, the reactors have to be purged thoroughly when being restarted.
It could therefore be helpful to provide a technical solution for preparation of ultrapure silicon, in which the above problems do not occur. More particularly, it could be helpful to enable very substantially continuous operation of a pyrolysis reactor without the regular interruptions mentioned.